this setting delete the closed transaction(failed or success).
Also, its better if you understand why those transaction still pending. For the oldest it could difficult discover the cause and about me, you can delete old the pending transaction most old than 30 days. Because its very probable those transaction are not usefull anc can create problems.
In IIQ a transaction could freeze in pending for some reason like upgrade, bad configuration etc…
If you continue to have transactions that do not close today, means the problem persist. Find and resolve the problem, before all.